In general, expandable ethylenically unsaturated polymer particles are prepared by the following processes.
(1) Styrene polymer particles are impregnated in a suspension medium by adding a volatile aliphatic hydrocarbon which does not dissolve or may slightly swell the styrene polymer particles; examples of such hydrocarbons include n-pentene, n-hexane and n-heptane.
(2) Styrene polymer particles are added to an aqueous suspension, emulsified with a small amount of solvent which dissolves the styrene polymer particles; examples of such solvents include benzene, toluene, xylene, carbon tetrachloride and tetrachloroethylene. The polymer particles are impregnated by adding an expanding agent which is gaseous under ordinary conditions, e.g. at room temperature and atmospheric pressure; examples of such expanding agents include propane, butane, vinyl chloride and iso-butane.
(3) Methacrylic acid ester copolymer particles consisting of from 99.5 to 95 % by weight of methacrylic acid ester and from 0.5 to 5 % by weight of .alpha.-methylstyrene, are impregnated by adding an expanding agent such as n-butane, n-heptane, cyclopentane and cyclohexane, at a temperature higher than the softening point of the said polymer particles and in the presence of a small amount of solvent such as halogenated hydrocarbons such as methylene chloride, 1, 2-dichloropropane and tri-chloroethylene.
(4) Ethylene polymer particles and a solution containing a cross-linking agent selected from dicumul peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexene-33, t-butyl hydroperoxide, cumene hydroperoxide and t-butyl peroxyisopropyl carbonate prepared in a solvent which can dissolve said polymer particles such as trichloroethylene, tetrachloroethylene, benzene, toluene and xylene, are admixed to form a suspension medium and said polymer particles are cross-linked and impregnated with an expanding agent such as n-butane, n-pentane, n-hexane and cyclohexane. The cross-linking and impregnation are carried out at the same time at the decomposition temperature of the cross-linking agent.
Expandable ethylenically unsaturated polymer particles obtained by means of the above processes are employed as materials for cellular shaped articles such as cups, packaging materials, insulation boards for refrigerators and structural components, etc.
The method of producing cellular shaped articles from expandable ethylenically unsaturated polymer particle is well known. Said polymer particles are expanded previously under heat to produce pre-expanded polymer particles, and the resulting pre-expanded polymer particles are then fed into a mold cavity having a plurality of perforations on the walls thereof, the shape of the mold cavity determining the shape of resulting articles. Said pre-expanded polymer particles are then heated, at a temperature above their softening point by means of suitable heating media, i.e., steam through the perforations, to let said polymer particles expand and fuse together to form cellular shaped articles, and after cooling the cellular shaped articles are removed from the mold cavity.
The surfaces of the expandable ethylenically unsaturated polymer particles have some restrictions in their production. That is, when impregnating an expanding agent, e.g. a volatile hydrocarbon and, especially, one having a high affinity for ethylenically unsaturated polymer particles such as n-pentane and n-hexane, into said polymer particles, the surfaces of said particles are softened and, as a result, the heat resistance of the particles is lowered. There is a tendency for the polymer particles to agglomerate, or to lump, (wherein "agglomerate" means the sticking together of a few particles and "lump" means the sticking together of agglomerated particles) when using a small amount of a solvent in the expandable ethylenically polymer particles in order to shorten the period of pre-expension. This is because the surfaces of the polymer particles become very soft by the action of the solvent, and therefore, the polymer particles tend to agglomerate during the pre-expanding step of the particles when raising the temperature to a level higher than the softening point of the polymer particles.
Pre-expanded polymer particles are generally transfered through a pipe to a storage hopper in order to be dried and aged, and are then transfered to a mold cavity through pipes. Subsequently, the pre-expended polymer particles are fed into the mold cavity through mold nozzles connected to the pipes. If the pre-expanded polymer particles include agglomerated particles or lumps, the particles tend to clog or plug the pipes and/or nozzles and, as a result, pre-expanded polymer particles are not transfered smoothly. Furthermore, feeding the mold cavity with such pre-expanded polymer particles can not be carried out effectively when the pipes and/or nozzles are clogged or plugged.
For the above reasons, expandable ethylenically unsaturated polymer particles are needed which do not agglomerate or lump during the pre-expanding step. In order to prepare expandable ethylenically unsaturated polymer particles which do not agglomerate or lump during the pre-expanding step, it is known that surfaces of ethylenically unsaturated polymer particles or expandable ethylenically unsaturated polymer particles are coated with a small amount of talc powder, wax, heavy metal soap such as zinc stearate and silicon oil; amorphous hydrated calcium silicon aluminate as taught in U.S. Pat. No. 3,444,104; and a fluid siloxane polymer as taught in U.S. Pat. No. 3,086,885. It is also known to coat the polymer particles with a small amount of kaolin clay, with a small amount of a mixture containing two particular types of surface active amides, namely, (1) a cationic normally solid, higher fatty acid amido- or resin acid amido- propyl-hydroxyalkyl quaternary nitrogen compound and (2) a nonioic, normally solid, polyethoxylated fatty acid amide, as taught in U.S. Pat. No. 3,301,812. Expandable ethylenically unsaturated polymer particles treated with the above coating agents can prevent agglomeration or lumping of the polymer particles in the pre-expanding step. However, it is difficult to get good cellular shaped articles from the above coated polymer particles when feeding them into the mold cavity and subsequently heating them to a temperature above the softening point of the polymer particles. Such difficulties arise, since the fusibility of the pre-expanded polymer particles are substantially decreased due to the effect of the coating agent which is primarily intended to prevent agglomeration or lumping of the polymer particles. Under such circumstances, expandable ethylenically unsaturated polymer particles are needed which have both the properties of not agglomerating or lumping during the pre-expanding step and providing good fusibility during the molding step.